Saturday, May 7, 2016

SNA Jr. Version II Part 1

I need to visit my daughter and help her with a couple remodeling projects around her house. I set the schedule so I can stop in Dayton for a couple of days at FDIM and decided to build an entry for the home-brew contest.  
With the success I had with the stand alone AD8307 power meter, I thought it was time to upgrade the SNA Jr. with a real power detector.  The SNA Jr. had started out with my version of the K6BEZ antenna analyzer and a need to check some filters I was working on for another project.  I replaced the SWR bridge circuit with a couple simple diode detectors and used the Arduino to compute  gain/loss in dB.  For such a simple project, I was really pleased with the results I was getting.  It was a vary small hand held instrument that I could use as a signal generator, SNA for testing filters.  With  the addition of a Return Loss Bridge I could check antenna SWR.  Then I added a simple pickup coil and a little software I also had some of the functionality of a Dip Meter.

It was very easy to edit the board layout for the original SNA Jr. to remove the diode detector and amplifier, and merge in the AD8307 circuitry from the power meter board.  The board house that I was using for the pilot run of club SWR meter project had a special going on.  Since I know of several people that would be interested in one of these boards I ordered 10 SNA Jr II boards.  It took about 2 1/2 weeks to get the boards, and am really pleased with the way they turned out.  The only issue I had was the spacing of the RF chokes I use to feed DC and signal into and out of the AD8307 circuit.  The ones I had are a little larger than the ones I had used in the stand alone power meter, and I had to bend the leads back on themselves to make them fit.  Other than that everything else works well.
In the original SNA Jr. I used a couple of 4.2 volt Li-ion batteries for power.  Only problem was that I had to remove the batteries to charge them.  In a rebuild of the SNA Jr. I used a 12V battery pack designed for use on security cameras.  This worked well, but I was not getting as long of operation on each charge as I had with the individual Li-ion batteries.

Looking around on eBay for something else I found some 2S Li-ion Lithium Battery 18650 Charger Protection Board Modules.
They are very small and take care of the charging and protection of 2 batteries in series,  there are also other versions available for different size battery packs.

I mounted a couple of battery holders , the charge controller board,  connectors and On/Off switch in the bottom half of the extruded aluminum box I bought for the project. The charge controller board is very small and after wiring it up I covered it with a piece of heat shrink and stuck it down beside one of the batteries. 

 I checked charging with a couple batteries and was very pleased with the way it operates.  I monitored charging current and voltage, it charged at about 1.6 amps with a wall wart power supply that put out 10.3 V. with no load.  While charging it put out around 8.6 V.,
and when charge was complete the current dropped to nearly 0 and voltage jumped up to the open circuit voltage. 

I built up one of the boards, and gave it a quick check to make sure everything was working.  Since I did not have the software modified to read the 8307, I checked the output of that circuit with a voltmeter.  

I got around the .25 V. I expected with no input and it jumped up to around .5V. when I touched the input with a screw-driver.  These are just about the same as I was getting with the circuit in the stand alone power meter.  Now that I know it is working I can build the shield box to cover the AD8307 circuitry

After adding the SMA connectors for RF output and input to the detector, I finished building a shield around the AD8307 circuitry using some .008" double sided circuit board.  That has become my favorite for building shields, easy to cut and solder, and stiffer than copper shim stock.  Here is a pictures of everything ready to be assembled, and one showing how tight everything fits in the box

Next to finish a little final fitting on the top half of the project box and mount everything, then on to the changes to the software.  Also thinking about adding a couple additional functions, such as directly computing VSWR instead of just Return Loss.  And add a wattmeter function, using a 40-dB tap to drop the power level down to that required by the AD8307.  Watch for updates, FDIM is less than two weeks away.



  1. Excellent upgrade work!
    Are project files available somewhere to mimic ?

    1. Not yet. Still doing a rewrite of the code to get ready for FDIM. Will have them available when finished.